The art of surveying involves the determination of unknown positions, surfaces or volumes of objects using measurements of angles and distances. In order to make these measurements, a surveying instrument frequently comprises an electronic distance measurement unit (EDM unit) which may be integrated in a theodolite, thereby forming a so-called total station. A total station combines electronic, optical and computer techniques and is furthermore provided with a computer or control unit with writable information for controlling the measurements to be performed and for storing data obtained during the measurements. Preferably, the total station calculates the position of a target in a fixed ground-based coordinate system. In, for example, WO 2004/057269 by the same applicant, such a total station is described in more detail.
A theodolite is schematically shown in FIG. 1, which figure will be used in the following to illustrate the typical errors that may occur in such surveying instruments. The theodolite 100 comprises a base with tribrach 105 mounted on a tripod 110, an alidade 115 mounted on the base for rotation about a vertical axis 120 and a center unit 125 mounted on the alidade for rotation about a horizontal axis 130.
The center unit 125 of the theodolite 100 comprises a telescope and optical elements for aiming at a target.
In the case of a total station, there may also be provided, in the center unit 125, an EDM unit which operates generally in the direction of the optical axis of the center unit, i.e. along the line of sight 135.
In a conventional EDM unit, a radiation beam is emitted from a radiation source, also called transmitter, as light towards the surface of a target (or scene) and the light beam that is reflected against the surface is detected by a receiver at the EDM unit, thereby generating a signal. Processing of the detected signal according to e.g. a time of flight measurement method or modulation phase shift measurement method enables the determination of the distance to the surface, i.e. the distance between the EDM unit and the target.
Accuracy of the measurements relies in part on mechanical stability of the geodetic instrument. In particular, the instrument performs accurate measurements if the various axis of the instrument are perfectly parallel or perpendicular to each other. For example, the axis of the EDM unit is preferably parallel to the axis of the center unit, i.e. the line of sight 135 of the total station 100.
As some deviation of the instrument axis from ideal positions normally occurs during the production or use of the instrument, accurate measurements relies on alignment and calibration of the various devices, optical channels and/or axis of the instrument. The optical channels may for instance be the line of sight and/or the transmitting and receiving optical channels of the EDM unit.
In the following, alignment and adjustment will be differentiated from calibration. Calibration is a process in which a parameter or instrument setting is measured and compared with a reference value. The difference between the measured value and the reference value is used to compensate the deviation using e.g. mathematical algorithms. The deviation may still physically be present but the influence of the deviation is cancelled or at least reduced. Alignment and adjustment are used to minimize the deviation by, e.g., mechanical, electrical, optical compensations.
For example, there may be a collimation error, which is illustrated in FIG. 1 by a vertical deviation εv and/or a horizontal deviation εh of the line of sight in vertical dimension (direction of a reference vertical axis) and horizontal dimension (direction perpendicular to a reference horizontal axis), respectively. There may also be a deviation εk of the horizontal axis 130 in a direction perpendicular to a reference vertical axis. This deviation εk represents a trunnion axis error of the total station 100, which deviation contributes to errors when aiming at targets out of the horizon.
Normally, calibration and alignment of the different optical channels and the axis of the instrument are performed at the production stage, e.g. during the manufacturing of the instrument and/or once the instrument is ready for sale. For instance, the axis of the EDM unit, in particular the relative positions of the transmitter and the receiver in respect to the line of sight may be adjusted (aligned). Further, the collimation errors as described above may be decreased by means of mechanical alignment. However, there may still be, after mechanical adjustment, a remaining deviation which can be measured and included in the instrument settings (calibration values) for compensation. The total station may also be subject to mechanical shock loads and temperature changes in order to test if the alignment and/or the calibration are sufficiently stable.
Further, a user may perform field calibration in accordance with instructions described in a user manual. For example, it is recommended to perform, for improving measurement accuracy, a manual calibration about fifteen minutes after starting the instrument, which corresponds to the period of time required by the instrument to warm up.
With a manual procedure, the user may for instance calibrate the collimation of the line of sight by aiming at a target according to a two-face measurement. The two-face measurement is based on the symmetry of the theodolite which allows reading of two angles when aiming at the same target. The direction angle, i.e. the combination of a horizontal angle and a vertical angle, is first read for a first face (face one) and, after rotating the instrument around the vertical axis by 200 gon and the center unit around the horizontal axis by 200 gon, the direction angle is read for a second face (face two). The difference between the horizontal angles (one read for the first face one and the other for the second face) reduced by 200 gon results in two times the collimation error in horizontal direction, i.e. 2εh. The difference between 400 gon and the sum of the vertical angles (one read for the first face (face one) and the other one for the second face (face two)) results in two times the collimation error in vertical direction, i.e. 2εv. The resulting deviations are then used to compensate angle readings during measurement sessions. Further, the user may calibrate the level point of the tilt sensor in the instrument by rotating the instrument around its vertical axis by approximately 200 gon.
However, a drawback of prior art methods and surveying instruments such as described above is that accuracy and reliability of the measurements are limited. Further, the requirement on mechanical stability is high, thereby increasing cost and time needed for the development of geodetic instruments.
Thus, there is a need for providing new methods and systems that would overcome these problems.